FIG. 1 is a longitudinal sectional view showing an example of a pressure sensor for detecting pressure in a combustion chamber of a conventional engine. The pressure lo sensor comprises a case 1 attached to an engine body (not shown), a pressure detecting part 2 comprised of a semiconductor strain gage and provided in the case 1, a signal line 3 connected at the tip end thereof to the pressure detecting part 2, a plug 4 for sealing the case 1, a diaphragm 5 provided on a lower end of the case 1, and pressure transmission liquid 6 having a high boiling point, sealingly accommodated in a pressure transmission chamber 7 defined between the diaphragm 5 and the pressure detecting part 2 within the case 1.
In the pressure sensor thus constructed, the pressure increase in the combustion chamber of the engine causes the diaphragm 5 to be deformed, so that the pressure of the pressure transmission liquid 6 is increased in connection with such deformation of the diaphragm 5, and then, such pressure increase of the pressure transmission liquid 6 is detected by the pressure detecting part 2. Based on the detection, a combustion state of the combustion chamber is judged so as to, for example, control an ignition timing for an ignition plug.
The conventional pressure sensor, however, has problems in which depending on changes in temperature of the circumstance e.g., the area, around the pressure sensor, i.e. in the ambient temperature, the pressure transmission liquid 6 is changed not only in volume but also in pressure, so that such changes of the pressure transmission liquid 6 adversely affect the readout of the pressure detecting part 2 as if the pressure changes in the combustion chamber occur.